2 research outputs found
Excited-State Proton Transfer on the Surface of a Therapeutic Protein, Protamine
Proton
transfer reactions on biosurfaces play an important role
in a myriad of biological processes. Herein, the excited-state proton
transfer reaction of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) has
been investigated in the presence of an important therapeutic protein,
Protamine (PrS), using ground-state absorption, steady-state, and
detailed time-resolved emission measurements. HPTS forms a 1:1 complex
with Protamine with a high association constant of 2.6 × 10<sup>4</sup> M<sup>–1</sup>. The binding of HPTS with Protamine
leads to a significant modulation in the ground-state prototropic
equilibrium causing a downward shift of 1.1 unit in the acidity constant
(p<i>K</i><sub>a</sub>). In contrast to a large number of
reports of slow proton transfer of HPTS on biosurfaces, interestingly,
HPTS registers a faster proton transfer event in the presence of Protamine
as compared to that of even the bulk aqueous buffer medium. Furthermore,
the dimensionality
of the proton diffusion process is also significantly reduced on the
surface of Protamine that is in contrast to the behavior of HPTS in
the bulk aqueous buffer medium, where the proton diffusion process
is three-dimensional. The effect of ionic strength on the binding
of HPTS toward PrS suggests a predominant role of electrostatic interaction
between anionic HPTS and cationic Protamine, which is further supported
by molecular docking simulations which predict that the most preferable
binding site for HPTS on the surface of Protamine is surrounded by
multiple cationic arginine residues
On the Molecular Form of Amyloid Marker, Auramine O, in Human Insulin Fibrils
Designing extrinsic fluorescence
sensors for amyloid fibrils is
a very active and important area of research. Recently, an ultrafast
molecule rotor dye, Auramine O (AuO), has been projected as a fluorescent
amyloid marker. It has been claimed that AuO scores better than the
most extensively utilized gold-standard amyloid probe, Thioflavin-T
(ThT). This advantage arises from the fact that AuO, in addition to
its usual emission band (∼500 nm), also displays a large red-shifted
emission band (∼560 nm), exclusively in the presence of human
insulin fibril medium
and not in the native protein or buffer media. On the contrary, for
ThT, the emission maximum (∼490 nm) largely remains unchanged
while going from protein to fibril. This otherwise unknown large red-shifted
emission band of AuO, observed in the presence of human insulin fibrils,
was tentatively attributed to a species formed upon fast proton dissociation
from excited AuO. It was proposed that because of the long excited-state
lifetime (∼1.8 ns) of AuO upon association with human insulin
fibrils, this fast proton dissociation from excited AuO could be observed,
which is otherwise not observed in buffer or native protein media,
owing to its very short excited-state lifetime (∼1 ps). Herein,
we show that despite the long excited-state lifetime of AuO in other
fibrillar media (human serum albumin and lysozyme), the new red-shifted
emission band at 560 nm is not observed, thus possibly suggesting
a different origin of the red-shifted emission band of AuO in human
insulin fibril medium. We convincingly show that this red-shifted
band of AuO (∼560 nm) could be observed under conditions that
promote dye aggregation, such as a premicellar concentration of surfactants
and polyelectrolytes. These AuO aggregates display strong emission
wavelength dependence of transient decay traces, similar to that for
AuO in human insulin fibril medium. Detailed time-resolved emission
spectral (TRES) measurements suggest that the AuO/premicellar surfactant
and AuO/human insulin fibril system share similar features, such as
a dynamic red-shift in TRES and an isoemissive point in the time-resolved
area-normalized emission spectra, suggesting that the characteristic
red-shifted emission band of AuO in human insulin fibril medium may
arise from AuO aggregates